That's Pixar's M.O. Unlike other animation studios pretty much tailoring a character to suit an actor (Shark Tale for example), Pixar usually goes for the actor best suited for the character without putting their names on the marquee. The major exception of course being Toy Story since they advertised the hell out of the lead actors for the first two films especially, but that's because they were just starting out as a company and what better way to get parents to take their kids to a movie than by luring them in with one of the biggest movie stars (Hanks) and one of the biggest network TV stars (Allen) for the time.
When you look at their resume, it's kind of rare when Pixar chooses a big name star to be a main character in their movies. For every Cars movie with Owen Wilson, you have a Brave where the biggest name involved is Emma Thompson and a Wall-E where Sigourney Weaver, the most famous actress in it, plays a small role. But I like that Pixar does that as opposed to taking the Dreamworks approach where they have to have multiple big celebrities in an animated film in order to sell it to parents.
